workloads/hwuitest: Do not try to process output if none present.

If the workload is abnormally terminated and no output is collected do
not attempt to process the output.

wa/workloads/hwuitest/__init__.py

@@ -69,12 +69,15 @@ class HWUITest(Workload):
         HWUITest.target_exe = self.target.install(host_exe)
 
     def run(self, context):
+        self.output = None
         self.output = self.target.execute("{} {} {} {}".format(self.target_exe,
                                                                self.test.lower(),
                                                                self.loops,
                                                                self.frames))
 
     def extract_results(self, context):
+        if not self.output:
+          return
         outfile = os.path.join(context.output_directory, 'hwuitest.output')
         with open(outfile, 'w') as wfh:
             wfh.write(self.output)